As is generally known, death, or necrosis, of living tissue cells occurs at temperatures elevated above a normal cell temperature. Further, the death rate of such heated tissue is a function of both the temperature to which it is heated and the duration for which the tissue is held at such temperatures.
It is also well known that the elevation of temperatures of living tissue can be produced with electromagnetic energy at frequencies greater than about 10 KHz.
It has been reported that some types of malignant cells may be necrotized by heating them to a temperature which is slightly below the temperature injurious to most normal cells. In addition, some types of malignant cells may be selectively heated and necrosed by hyperthermia techniques because masses of these malignant cells typically have considerable poorer blood flow and thus poorer heat dissipation properties than does the surrounding normal tissue. As a result, when normal tissue containing such malignant masses is heated by EMR (electromagnetic radiation), the resultant temperature of the malignant mass may be substantially above that of surrounding healthy tissue.
It has been determined that most malignant cells have a relatively limited temperature range in which hyperthermia is effective in causing tumor necrosis. Above a threshold temperature of about 41.5 degrees Celsius, substantial thermal damage occurs even in those types of malignancies which have a greater sensitivity to temperature than do normal cells. In fact, at temperatures just below this threshold, growth of some types of malignancies may be stimulated. At temperatures above 45 degrees Celsius thermal damage to most normal cells occur when exposed more than 30 minutes. A discussion of hyperthermia in the treatment of cancer is contained in "Physical Hyperthermia and Cancer Therapy" by J. Gordon Short and Paul F. Turner in the "Proceeding of the IEEE", Vol. 68, No. 1, January, 1980 herein incorporated by reference.
Typically, EMR heating of body tissue is accomplished by holding an EMR radiator, or applicator, adjacent to, or against, exterior portions of a body, the EMR then penetrating and heating subsurface portions of tissue. However, significant amounts of energy are absorbed by surface or epidermic layers which may have to be cooled in order to prevent damage thereto by overheating.
The amount of penetration, or the depth of which EMR causes effective heating, is dependent upon the frequency of radiation.
For example, in accordance with an article by A. W. Guy, et al, published in proceedings of the IEEE, Vol. 63, No. 1, January, 1974 entitled "Therapeutic Application of Electromagnetic Power", the depth of penetration in the human muscle and fat at 100 MHz is 6.66 cm and 60.4 cm, respectively, while at 915 MHz the depth of penetration is only 3.04 cm and 17.7 cm, respectively.
In general, the lower the EMR frequency, the larger the applicator must be in order to effectively radiate the energy into the tissue and, as a result, applicators for radiating EMR below one gigahertz tends to be large in size and cumbersome to handle. Additionally, such applicators are not configured to selectively heat tumors of various sizes and shapes located well beneath the surface layer of the body being irradiated. Further, tumors, or other selected areas, shielded by a layer of boney tissue such as a skull, are difficult to effectively heat deeply with externally applied EMR.
Invasive EMR applicators, that is, radiators which can be inserted into body tissue to the region either within or adjacent to malignant tumors, or other localized growths, for selective heating thereof, may cause nonuniform heating or "hotspots" at or nearest the surface of such applicators because of nonuniform field distributions. Such unwanted "hot spotting" is more likely to cause serious overheating when such invasive applications are operated at higher power levels in order to heat large localized growths using a single applicator. Such growths may be many times the size of the radiating area of an invasive type applicator. Examples of invasive EMR applicators are disclosed in U.S. Pat. Nos. 4,448,198 and 4,669,475 entitled "Invasive Hyperthermia Apparatus and Method" which disclose the application of several invasive type applicators and methods of using the apparatus to effectively heat relatively large localized areas within living body tissue, without significant hot spotting at or about the applications.
The localized areas included those located well beneath surface layers of the body tissue. According to the method disclosed, the heating of these local regions within living body tissue were accomplished with a radiation source providing electromagnetic radiation to several applicators. Each of the applicators were adapted for insertion into the body tissue and for radiating EMR energy therein.
In addition to the applicators, several invasive temperature probes were disclosed for monitoring temperatures within a target growth and also the normal tissue just outside the target growth.
The improved applicators provided an applicator for insertion into body tissue for radiation of EMR and for the detection of temperature therein. This enables the system to provide radiation to the applicators in accordance with the sensed temperatures to control the heating of localized areas of body tissue. This also included a system for controlling radiation provided to localized areas of body tissue by calibrating the applicator temperature sensor to determine the portion of measured temperature resulting from the heating of the applicator.
An article by B. Stuart Trembly et al. entitled "Control of the SAR Pattern Within an Interstitial Microwave Array through Variation of Antenna Driving Phase", published in IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-34, No. 5, May 1986, also demonstrated the use of peripheral applicator phase change in a continuous manner as was contained within U.S. Pat. No. 4,448,198. This continuous phase variation is shown to provide improved uniformity over the fixed phase examples of coherent arrays. Further, the region of large SAR (specific absorption rate) can be shifted within the array by changing the driving phases so as to create phase coherence at a point other than the center.
The disadvantage of the prior art references is that they contain substantial limitations to control the temperature of the region of large SAR. Clinically it is not very satisfactory to place so much heat in the central zone of an implant with much less along the margins which is a characteristic of large coherent phased arrays. Usually, there may be impaired central tumor blood flow (in necrotic zones) which would enable further central temperature elevation above the perimeter zones. Although it is possible to moderate the power to each applicator of this array, there would not be a change of this basic central heating phenomena and the clinician would be faced with a difficult decision during treatment: either central temperature would be allowed to approach 50 degrees Celsius to enable perimeter heating to 42 degrees Celsius or non-therapeutic temperatures would encompass the implant perimeter. Above about 45 degrees Celsius, healthy tissue can be damaged. To partially overcome excess heating near the antenna, more antennas had to be inserted into the patient. The ideal method is to both minimize the required number of antennas, and to maximize the uniformity.